Every electronic circuit is designed to operate off of some supply voltage, which is usually assumed to be constant. A voltage regulator provides this constant DC output voltage and contains circuitry that continuously holds the output voltage at a regulated value regardless of changes in a load current or input voltage. A linear voltage regulator operates by using a voltage controlled current source to output a fixed voltage. A control circuit must monitor the output voltage, and adjust the current source to hold the output voltage at the desired value.
One of the problems that a wide range input voltage, such as 3v to 20v, places on a linear voltage regulator is thermal stress when operating at high input supply voltage while providing a low output voltage. This is further compounded when the linear regulator is only one aspect of the total chip functionality, and the total thermal budget cannot be used up by the Linear Regulator. Most of the thermal stress is on the current source and the exact magnitude of the problem is very application specific. The easiest way to control the problem is to control the current source by allowing it to be either internal or external. Existing linear voltage regulators are unable to be configured with either internal or external current sources.